A variety of techniques have been devised to reduce the torque variability or ripple in stepper motors. One such technique modifies the drive current waveforms that energize the windings. For example, in U.S. Pat. No. 4,634,949 to Golladay, two groups of phase windings are driven by sinusoidal waveforms in quadrature with each other that have different first and second voltage amplitudes. Other techniques involve breaking rotational symmetry by displacing the positions or modifying the relative dimensions of one or more groups of poles. For example, U.S. Pat. No. 4,739,201 to Brigham et al. shows how one can reduce any given harmonic of the torque/angle curve by displacing a first set of rotor or stator pole teeth from their “normal” position relative to a second set by a displacement angle that has been calculated to cancel the harmonic generated by one set with that generated by the other set. U.S. Pat. Nos. 5,852,334 and 6,060,809 to Pengov employ a rotor with alternately wide and narrow pole faces. Still other techniques effectively average the magnetic field's influence on torque by having different numbers of rotor and stator poles and/or teeth with correspondingly different pitches and rotational offsets with respect to one another (cf., U.S. Pat. Nos. 4,423,343; 4,647,802; 4,675,564; 5,157,298; 5,309,051). Electronic and mechanical dampings have been used to improve the smooth motion. In U.S. Pat. No. 6,008,561 to Tang, a motor is provided with auxiliary damping windings which are coupled to form a closed current loop that absorbs energy from or provides energy to the phase windings. In U.S. Pat. Nos. 5,889,347 and 6,078,122, Tang et al. provide fractional-pitched coils that allow both self- and mutual-inductions to contribute to the torque output.
The present inventor has also contributed to the advancement of stepper motors, as exemplified in prior U.S. Pat. Nos. 4,638,195; 4,910,475; 6,114,782; and 6,597,077. In the last of these aforementioned patents ('077), Lin et al. describe a two-phase step motor with bifilar windings around the stator poles, which are connected to a two-phase driver in a manner, called a T-connection different from conventional series and parallel stator coil connections, that maximizes torque at medium speed operation. In particular, four stator coils are wound around the stator poles in a bifilar winding pattern with coils wound around different groups of stator poles are connected in series. In one set, both coils are connected in a forward sense around the stator, while in the other set the two coils are connected in opposite senses around the stator. The T-connection also smoothes stepping motion and reduces vibrations compared to the conventional parallel or series connections for stepper motors.
Traditional half-stepping allows the motor to stop alternately at one-phase ON and two-phase ON positions. The phase angles for these positions are 0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°, then again back to 0°. The motor settling time characteristics are different at one-phase ON and two-phase ON. The step time profile from every controller assumes that the settling time character of each step is the same. The erratic motion occurs when the step time is not synchronous with the rotor position.
An object of the present invention is to provide an improved half-stepping motor for smoother motion and low noise.